The present invention relates to a process for the preparation of tert.-olefins by cleavage of their alkyl ethers over specific catalysts which very sharply suppress the formation of the undesired dialkyl ethers and at the same time have a long service life.
It has been known for a long time to obtain tert.-olefins from distillate cuts (for example from product streams of crackers) in a pure form in such a way that they are first selectively converted with alkanols into their alkyl ethers and these alkyl ethers are then cleaved again after they have been separated from the remaining distillate cut. Acidic cation exchangers, for example styrene/divinylbenzene resins containing sulphonic acid groups, have been disclosed as cleavage catalysts, apart from other catalysts.
All cleavage catalysts form, from the alkanol liberated in the cleavage, the respective dialkyl ether which is more volatile than the underlying alkanol and hence accompanies and contaminates the tert.-olefin which is separated off as the top product by distillation. To a particularly great extent, this applies to dimethyl ether, which arises in the cleavage of methyl tert.-butyl ether (MTBE) or tert.-amyl methyl ether (TAME) and which contaminates the desired i-butene or tert.-amylene respectively.
In attempts to suppress the formation of these dialkyl ethers, only catalysts were obtained which lost their activity and selectivity with respect to the production of pure tert.-olefines to such an extent after a few days, frequently only a few hours, that they are completely unsuitable for low-cost continuous processes. It therefore appeared as though it was impossible to resolve the "dilemma", namely having to accept the disadvantage of the extensive contamination of the tert.-olefin by the dialkyl ether or having to accept the disadvantage of the intolerably short service lives of the catalyst.